the sr102 is poor kit, however I would expect it to still have equal performance at peak as well as off peak, however it is possible something on that device was causing those spikes, and if the BT modem has fixed the spikes and your performance then I say keep the BT modem in place. 
Putting the BT OR in instead of the SR102 has fixed the spikes (
My Broadband Ping) but speed on the 1x is still showing the same result I.e. Dropping down to 20mb at certain points, you can retest 20 minutes later and get a better result.
Speed of.me and speedtest don't show the same problem (both showing 64mb) but they do not detail how they perform the tests.
Line is clean according to the fibre engineer and he was at the house for an hour testing the line. Sky fibre specialist checked the exchange and at peak they are under 90% utilisation for their back haul.
That only leaves the cabinet but the fibre engineer said they would have got warnings but their aren't any so I am at a complete loss as their is no guarantee that jumping to Pulse8 will improve matters if it s not a congestion issue.
Scratching my head on this one!
plusnet Unlimited Fibre (FTTC) > Sky Fibre Pro Unlimited. 15ms Ping, Sync ~ 68.93/18.83Mbps